Copper foil for printed circuits generally is laminated and bonded to the base of a synthetic resin or the like at high temperature and high pressure, screen printed with necessary circuit patterns using a resist to form objective circuits, and then is etched with an etchant such as a cupric chloride solution to remove unwanted portions. Finally, desired elements are soldered in place, and in this way various printed circuit boards for electronic devices are fabricated. Qualitative requirements for the copper foil for printed wiring boards differ depending on the side of the foil, namely the matted side to be bonded to the resin base and the opposite, shiny side.
Requirements for the matte side to which the present invention pertains may be boiled down to:
(1) Adequate resistance to peeling from the base even after high-temperature heating, wet treatment, soldering, chemical treatment or the like (peel strength): PA1 (2) No possibility of oxidative discoloration during storage (corrosion preventiveness); PA1 (3) Freedom from so-called lamination spots that can be observed after lamination to the base followed by etching (hydrochloric acid resistance); and PA1 (4) No dusting after etching (prevention of dusting).
Of these requirements, a sufficiently high peel strength is the most imperative for the surface to be bonded to a base.
To enhance the bond strength between the copper foil and the resin base, a roughening-treated layer consisting of numerous small protuberant copper electrodeposits is formed on the surface of the copper foil, particularly a rolled copper foil, to be bonded to a base. For the roughening of an electrolytic copper foil, which already has uneven surface with knurls produced in the electrolytic course, many protuberant copper electrodeposits are formed on and around the tops of the individual knurls to increase and reinforce them.
An effective roughening treatment for electrolytic copper foils is disclosed in the literature including Japanese Patent Application Publication Nos. 38053/1979 and 39327/1978 etc. It involves electrolysis in an acidic copper electrolytic bath containing arsenic, antimony, bismuth, selenium, or tellurium at about critical current density. Practically, arsenic acid is often added to the electrolytic bath. In this way, a number of protuberant copper electrodeposits are formed on and around the individual knurls of the raw electrolytic foil, thus effectively further roughening the surface for greater bonding strength.
In the case where arsenic takes part in the electrolysis, several hundreds of parts of arsenic per million is mixed into the electrodeposits. This poses a serious environmental or health problem when the copper foil thus treated is recycled or otherwise exploited or when an etching solution in which arsenic has dissolved out is disposed of.
Methods aimed at roughening without employing such a toxic element have hitherto been proposed. Among them are the use of a bath containing a trace amount of a benzoquinoline (Pat. App. Pub. No. 41196/1981), treatment with a bath containing molybdenum, vanadium or both (Pat. App. Pub. Nos. 56677/1987 and 56678/1987), and pulse plating (Pat. App. Public Disclosure Nos. 17597/1988 and 164797/1983). None of those methods have proved fully satisfactory yet in respect of the peel strength, dusting or other properties of the treated surfaces.